This invention relates to optical monitoring systems.
The transmitter of many optical communications systems, especially those employing semiconductor lasers, typically includes a feedback loop to stabilize the laser against changes in its output due to aging, temperature variations and the like. Various schemes have been proposed. In one, the light output from one mirror of a semiconductor laser is coupled to an optical fiber for transmission purposes whereas the light output from the opposite mirror is detected by a photodiode. The electrical output of the photodiode is used in the feedback circuit to alter the drive current to the laser to compensate for variations in its light output. See copending application, Ser. No. 667,961 of I. Camlibel et al. filed on Mar. 18, 1976 and issued as U.S. Pat. No. 4,119,363 on Oct. 10, 1978, and assigned to the assignee hereof. This technique suffers from the disadvantage that the light outputs from the opposite mirrors of a semiconductor laser are not always identical; i.e., they do not necessarily track one another. Another scheme overcomes this problem by employing only the light emitted from one mirror into an optical fiber. The fiber is bifurcated into two segments having spaced, parallel beveled surfaces. A portion of the light incident on the beveled surfaces is reflected out of the fiber to a photodiode located in a feedback circuit as before. Unfortunately, this arrangement is sensitive to changes in the modal distribution of the light entering the fiber caused by shifting of the position of the lasing filament in the active region of the laser or of the fiber with respect to the active region. As described in copending application Ser. No. 861,288 of M. DiDomenico, Jr. et al. filed on Dec. 16, 1977 and issued as U.S. Pat. No. 4,165,496 on Aug. 21, 1979, and assigned to the assignee hereof, this problem can be alleviated by proper choice of the bevel angle or by positioning a steady state mode mixer between the laser and the beveled surfaces of the light tap.